1. Technical Field
The present development relates to management of fuel evaporative emissions.
2. Background Art
A typical automobile has a carbon canister coupled to a vent of the fuel tank. Activated carbon pellets in the carbon canister strip fuel vapors from the gases displaced by fuel entering the fuel tank during a refueling operation. The gases that have been stripped of fuel are vented out of the carbon canister to the atmosphere. Additionally, due to natural daily temperature changes (diurnal cycle) to which the vehicle is subjected when parked, the fuel is heated and cooled, thereby vaporizing and condensing fuel, respectively. If the vehicle fuel and fuel tank temperatures increase by 30° F., the volume of the gases above the fuel in the fuel tank expands by about 25 liters for a typical automotive fuel tank. By having a vent from the fuel tank into the carbon canister, fuel vapors from the gases expanding out of the fuel tank are adsorbed on the activated carbon. Such processes are referred to as a vapor recovery mode.
Eventually, the activated carbon pellets become saturated and can adsorb no additional fuel. To avoid saturation of the carbon canister and subsequent release of fuel vapors, the carbon canister is periodically purged during engine operation. The carbon canister has a port coupled to the intake of the engine with a valve between the carbon canister and the engine. When the engine is operating at a favorable condition for purging the carbon canister, the valve is opened and fresh air from the atmosphere is drawn into the carbon canister, with the fresh air desorbing fuel vapors from the activated carbon pellets. The air with fuel vapor is inducted into the engine and combusted. This is referred to as a purging mode.
A problem encountered in some modern vehicles is that the engine is operated infrequently at a condition which is favorable for purging the carbon canister. For example, with a plug-in hybrid electric vehicle (PHEV), the vehicle may be propelled solely under electric operation, particularly at low torque operating conditions. During such operation, the carbon canister cannot be purged without otherwise unnecessary operation of the internal combustion engine. Furthermore, when the internal combustion engine is operating in a PHEV, it tends to be operated at higher torque operating conditions with associated lower manifold vacuum preventing the carbon canister from purging as rapidly as desired. This is because the carbon canister relies on intake manifold vacuum to draw the fresh air through the carbon canister and into the intake manifold. Thus, the opportunities for purging the carbon canister are lessened both because the engine is operated less often, and because the engine is more likely to operate with a low manifold vacuum when the engine is being operated.
As recognized by the present disclosure, PHEVs are not the only vehicle systems that encounter difficulties in purging the carbon canister to manage evaporative emissions. Engines with pressure-charging devices, such as superchargers or turbochargers, may have a smaller displacement than a naturally-aspirated engine sized for the same vehicle. Pressure-charged engines operate at a higher manifold pressure (or lower manifold vacuum) than a naturally-aspirated engine. Consequently, there are also concerns with fully purging the carbon canisters coupled to these engines. Such engines may include a gasoline turbocharged direct-injection engine (GTDI), for example. Additionally, any engine employing measures to reduce pumping losses, such as using variable valve timing (VVT), lean burn, stratified charge, homogeneous-charge compression-ignition (HCCI), etc., also encounters difficulty in having sufficient operation at high manifold vacuums to purge the carbon canister as desired.
When a canister becomes saturated, no additional fuel vapors can be stripped from gases passing through the carbon canister and any fuel filling or expansion of gases in the fuel tank due to temperature changes would result in displaced gases which contain fuel vapors being unintentionally released to the atmosphere. A particularly troublesome situation occurs when a vehicle is parked for multiple days. The vapors released from the tank into the carbon canister during the hot portion of the day are processed in the carbon canister. At night, the gases contract and pull in fresh air into the system. After a number of such cycles, the carbon canister may become saturated and successive cycling may result in release of fuel vapors.
One alternative is to provide a fuel vapor recovery system that can withstand a pressure due to a temperature rise and a vacuum due to a temperature decrease. Such a system requires more costly components: steel fuel tank (compared to plastic tanks commonly used), stronger construction of the carbon canister, and fittings/connectors throughout the system that seal under both pressure and vacuum.